Supply of electricity from providers such as power stations, to consumers, such as domestic households and businesses, typically takes place via an electricity distribution network. FIG. 1 shows an exemplary electricity distribution network 1 comprising an electricity transmission grid 100 and an electricity distribution grid 102. The transmission grid is connected to generating plants 104, which may be nuclear plants or gas-fired plants, for example, from which it transmits large quantities of electrical energy at very high voltages (in the UK, for example, this is typically of the order of 204 kV; however this varies by country), using power lines such as overhead power lines, to the distribution grid 102. The transmission grid 100 is linked to the distribution grid 102 via a transformer node 106, which includes a transformer 106 which converts the electric supply to a lower voltage (in the UK, for example, this is typically of the order of 50 kV; however, this varies by country) for distribution in the distribution grid 102. The distribution grid in turn links, via substations 108 comprising further transformers for converting to still lower voltages, to local networks such as a city network 112 supplying domestic users 114, and to industrial consumers such as a factory 110. Smaller power providers such as wind farms 116 may also be connected to the distribution grid 102, and provide power thereto. Electric power is typically transmitted through all parts of the electricity distribution network in the form of a sinusoidal alternating current (AC) wave.
Electric power consuming devices connected to the network at, for example, the site of a domestic user 114, act as a load on the network, drawing power therefrom. The load provided by each such device is typically not purely resistive, but includes a reactive element, due to capacitive and/or inductive elements in the devices. The reactive component of the load may be particularly large in devices such as electric motors and transformers, which have a high inductance, and devices which have a high capacitance. Such devices produce a reactive current component which flows at ±90° to the voltage; this results in a phase shift in the current flowing in the network with respect to the voltage.
The term “reactive power” is used herein to refer to the product of a reactive current component and the voltage flowing at a given location of the network. This reactive power results in no net energy transfer to the device, but it does have an effect on the network and on power suppliers, as described below. The term “real power” is used herein to refer to the rate of electrical energy consumption by a power consuming device. The term “power factor” is used herein to refer to the ratio of real power to the vector sum of the real power and reactive power.
Whilst the effect of the reactive load of an individual device on the current-voltage phase difference in the network may be small, the cumulative effect from multiple devices can be significant. The larger the current-voltage phase difference becomes, the greater the size of the current that must be supplied to a device in order to supply a given real current (i.e. the current component flowing in phase with network voltage), and therefore a given amount of real power. Further, energy losses in the network, due to e.g. heating of electric power lines, depend on the total current flow, irrespective of whether the current flow is real or reactive. Accordingly, such phase differences effectively increase the size of the total current that must be generated and supplied by a power provider in order to meet the demands of its customers; this places an economic burden on the power supplier, increasing the cost of electrical power generation. Similarly, the amount or resources consumed by the power provider in order to supply a given amount of power to a consumer is increased, which may have undesirable environmental consequences.
Further, network elements such as transformers and power lines are dimensioned according to total the size of the current (whether real or reactive) flowing in the network; their operation is therefore adversely effected by any reactive current flow (due to thermal losses and so on).
Conventionally, efforts to reduce the current-voltage phase difference of power flowing in such networks have focussed on minimising the reactive power contribution of, and/or creating an appropriate amount of compensating reactive power at, large scale power suppliers, and creating compensation for reactive power at transformer stations within the electricity distribution network. For example, a power station may use banks of capacitors and/or inductors, independently or under instructions from the network operator, to adjust the reactive power contribution of the power station. However, reactive power compensation is effective only at short distances (due to e.g. thermal losses), and, furthermore, the current-voltage phase difference may vary significantly from location to location within the electricity distribution network; this means that reactive power compensation at a small number of large scale power providers does not effectively compensate for localised current-voltage phase differences.
Some large scale consumers of reactive power may also use some means of minimising their own reactive power contribution to the network by using supplementary devices to compensate for the reactive power they generate, such as switched capacitors or an unloaded synchronous motor; indeed, some power providers encourage industrial consumers (such as factories) to contribute less reactive power by charging for reactive power contributions in addition to real power consumption. These methods all focus on minimising the contribution of individual devices to a current-voltage phase difference in the network.
U.S. Pat. application US2009/0200994 describes a distributed system of renewable energy sources each including circuitry for generating reactive power on demand. Each of the renewable energy sources is in communication with a central controlling “Network Operations Centre”, which remotely controls reactive power production by renewable energy sources. The Network Operations Centre receives a request from a utility company (i.e. a power provider) for a required amount of reactive power; in response the Network Operations Centre calculates an optimum reactive power contribution required from each of the renewable energy sources under its control to produce the necessary compensation, and sends commands to the renewable energy sources accordingly. This provides a method of actively compensating for current-voltage phase differences that may be present in the network. However, the system of US20090200994 requires central control and cannot react to more localised changes in local network voltage-current phase differences.
It is an object of the present invention to at least mitigate some of the problems of the prior art.